Field of the Invention
The present invention relates to x-ray and gamma-ray imaging, and more specifically, it relates to techniques for lowering the dose required in such imaging techniques.
Description of Related Art
In conventional 2-D x-ray/gamma-ray imaging the patient or object is illuminated with a wide field of x-rays or gamma-rays and the transmitted signal is recorded on a 2D film or array of detectors. Variations of density within the object cause variations in transmission for the penetrating radiation and these variations appear as shadows on film or a detector array. The dynamic range of this imaging technique is determined by the response function of the detector system. In addition all parts of the object see the same input flux (photons per unit area) and the total dose impinging upon the object is set by the area of the object and the flux required to penetrate the most dense region of the object, i.e., the flux required to resolve the structures of interest within the object. In this imaging modality, the entire object sees a high dose.
Suggestions for pixel by pixel feedback imaging have been made previously in which a rotating anode, bremsstrahlung source is used in place of a laser-Compton source. In this case, upon accumulation of a threshold quantity of photons at the detector, a signal is sent to either disable the current to the anode or to physically block the x-ray beam. This approach suffers several drawbacks in relation to the invention of this disclosure.
a) Rotating anode sources are CW or quasi-CW devices and neither interrupt methods mentioned above are instantaneous. Thus, there will be dose accumulated while the source is being shut down or is being physically blocked. On the other hand, in the case of a laser-Compton x-ray source (LCXS) or laser-Compton gamma-ray source (LCGS) x-rays or gamma-rays are produced for each interaction of a laser pulse with an electron bunch. If the signal from the detector to divert the laser pulse is fast compared to the time interval from one laser pulse to the next and the electro-optic switch operation is fast compared to the time interval from one laser pulse to the next, then the x-ray or gamma-ray source may be turned off completely before additional exposure occurs.
b) Rotating anode devices operate with beams of electrons impinging upon the anode material at a constant rate. Interruption of the electron beam current, can change the electromagnetic environment around the anode and the thermal loading of the anode material. Re-initiation of electron beam does not necessarily instantaneously produce the same electron beam focus or x-ray flux as that occurring during steady state operation. On the other hand, in the case of the LCXS by electro-optically diverting the laser pulses from interacting with the electron beam, one does not change the electron beam dynamics of the electron beam used in a laser-Compton source. The electron beam may remain on and operational even without producing x-rays or gamma-rays if there are no laser photons. A simple change of the electro-optic switch that returns the laser photons to the laser-electron interaction region will produce a laser-Compton source that is identical to that used to image the previous pixel.
c) Rotating anode sources are not well suited to production of highly-collimated beams of x-rays or gamma-rays. They produce light in all directions and can only produce collimated beams by passage through narrow apertures that in turn greatly reduce their flux. LCXS and LCGS devices intrinsically produce narrow beams of photons. Effectively all of the LCXS may be used for single-pixel feedback imaging while only a small portion of the output from a rotating anode source may be used in this manner.